• Develop an understanding of the history of the science of light.
• Identify light as part of the electromagnetic spectrum.
• Recognize that light travels in a straight path that can be represented by rays.
• Explore light as a form of radiant energy. Light energy can become heat energy and heat energy can become light energy.
• Examine the discrete particles of light called photons.

• electromagnetic spectrum
• emission
• frequency
• light
• photon
• ray
• reflection
• wave
• wavelength

• Class One: access to textbooks and encyclopedias
• Class Two: powerful flashlight
• Class Three: 2 ice cubes, plastic tray, black construction paper, white construction paper, 2 flashlights
• Class Four: No special materials required.

The unit is divided into four class periods. Classes Two and Three can be combined. Allow approximately 20 minutes for each online SimLibrary activity.

1) Have students use a textbook or an encyclopedia to build a timeline of the history of theories about the nature of light since the 1600s. Working either as a class or individually, have students place the following individuals on the timeline, together with a 2-3 sentence summary of each theory about the nature of light.

• Isaac Newton
• Christian Huygens
• Thomas Young
• James Clerk Maxwell
• Max Planck
• Albert Einstein
• Philipp Lenard

2) Explain that light is an electromagnetic phenomenon, part of the electromagnetic spectrum. The spectrum is made up of waves of different wavelengths and frequencies.

Light is the range of the electromagnetic spectrum that stimulates the retina and enables sight.

3) If students have already studied waves, ask them to name some general characteristics of waves.

Light exhibits characteristics typical of waves: reflection, refraction, interference. Some of these characteristics are discussed in Unit 3: Light and Matter.

1) Have all the students face the front of a slightly darkened classroom. Ask a student to determine where you are standing, without turning around to look. Then from the back of the classroom, turn on a powerful flashlight and aim it over the heads of the students to the front wall. Turn off the flashlight, move to another location, and repeat. Repeat for several locations. Ask the students how they were able to determine the location of the light source without actually seeing it. Then create a little chalk dust and shine the flashlight through it. Students should be able to see the straight-line path of the light.

2) Summarize that light travels in a straight line. Ask students to use this fact to explain a shadow. Would shadows exist if light traveled in a curved path or could turn corners?

3) Continue the discussion: A ray is a straight line that represents the path of a narrow beam of light, usually from the source. We use a ray model of light when studying phenomena like reflection and refraction. Even though the model ignores the wave nature of light, it accurately represents the path of light. Here is a typical ray diagram showing light reflection:

1) Perform the following demonstration for the class:

• Place two ice cubes on a plastic tray.
• Cover one ice cube with a piece of black (or dark-colored) construction paper. Cover the other ice cube with a piece of white (or light-colored) piece of construction paper.
• Hold a flashlight over each piece of ice. Make sure that they are equidistant from the cubes. Watch to see the rate at which each ice cube melts.

Ask students to explain the demonstration using the terms "light" and "heat." (Explanation: The light turns to heat energy when it is absorbed into the paper. The dark paper absorbs more light, so it heats more, and the ice melts more quickly. The white paper reflects more light, so the paper heats less quickly.)

2) Ask students to name 5 light sources that provide significant amounts of heat. Record them on the chalkboard.

3) Ask students to give 3 examples of devices that use light to generate electricity (all solar-powered devices, like calculators, hot water heaters, etc.).

4) Explain that electromagnetic waves transmit radiant energy. Microwaves, which are also on the electromagnetic spectrum, heat food by exciting the electrons in the food's atoms. As the electrons move between energy levels, they generate heat. Light works in a similar way, so that sunlight warms us.

1) Define light emission: The giving off of light. Ask students to predict what is occurring in an atom that enables it to emit light.

2) Continue the discussion: When electrons drop from a higher to a lower energy level, they emit a pulse of electromagnetic radiation called a photon. These photons are light particles. Thus, light is both a wave and particles. Albert Einstein was the first to recognize this dual nature of light. He was awarded the 1921 Nobel Prize for Physics for the discovery.